This invention relates to electrophoretic information displays (EPID) in general and more particularly to apparatus which operates in conjunction with an EPID display enabling such a display to operate with grey scale capability.
The prior art is replete with a number of various patents and articles concerning electrophoretic displays. Such electrophoretic displays have been widely described and disclosed in the prior art and essentially the assignee herein, Copytele Inc. of Huntington Station, N.Y. has developed an electrophoretic display which has an image area of approximately 11.times.81/2 inches and is designed to be used either as a separate display or to be combined with other displays. The company has the ability to combine as many as four such displays to create larger area displays. The information on such displays can be changed either locally or remotely and can be viewed at an angle of nearly 180 degrees. Such displays have extremely high resolution and can accommodate over 160,000 pixels within an image area of approximately 2.8 inches diagonally. In regard to such displays, reference is made to U.S. Pat. No. 4,655,897 issued on Apr. 7, 1987 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS to Frank J. DiSanto and Denis A. Krusos and assigned to Copytele Inc., the assignee herein. In that patent, there is described an electrophoretic display panel which includes a planar transparent member having disposed on the surface a plurality of vertical conductive lines to form a grid of wires in the Y direction. On top of the grid of vertical lines, there is disposed a plurality of horizontal lines which are positioned above the vertical lines and insulated therefrom by a thin insulating layer at each of the intersection points. Spaced above the horizontal and vertical line pattern is a conductive plate. The space between the conductive plate and the X and Y line pattern is filled with an electrophoretic dispersion containing chargeable pigment particles. When a voltage is impressed on the X and Y lines, pigment particles which are located in the wells or depressions between the X and Y pattern are caused to migrate towards the conductive plate and deposited on the plate in accordance with the voltage applied to the X and Y conductors. There is described in that patent an electrophoretic dispersion suitable for operation with the display as well as techniques for fabricating the display. Hence, in this manner the displays can be fabricated to contain large effective display areas while being relatively thin. These displays are capable of high resolution and relatively low power consumption. As indicated, the above noted patent and others include information concerning the fabrication, operation and resolution of such displays. As explained in U.S. Pat. No. 4,833,464 issued on May 23, 1989 and entitled ELECTROPHORETIC INFORMATION DISPLAY (EPID) APPARATUS EMPLOYING GREY SCALE CAPABILITY to F. J. DiSanto, et al., it is a problem with such displays to provide grey scale capability. Grey scale capability is a well known term of the art and has been utilized for example in regard to the description of television receivers and various other types of data presentation, such as facsimile and so on. In the case of television receivers, the response of the receiver can be visually determined by means of typical test patterns such as those test patterns that were previously transmitted and displayed when, for example, a television station goes off the air. Various television stations frequently transmit such a pattern for the convenience of service technicians and so on. The pattern apart from showing correct linearity, for example, also shows correct reproduction of the background shading which can indicate proper frequency response. The correct reproduction of the five color shades in the center target area of the test pattern indicates proper mid frequency responses. As one can ascertain such test patterns are associated with grey scale capability, namely with the display of various grey levels as located between black and white. Such grey scale capability is a desirable feature in conjunction with any type of display. An electrophoretic display either presents a black or white type of representation of an image which is conveniently referred to as dark or light. Basically the color of the image is a function of the color of the pigment particles and the color of the suspension that they are suspended in. The display may be black and white, yellow and black and so on. There are a wide variety of many potential color combinations which can be employed in regard to such displays. The above noted U.S. Pat. No. 4,833,464 describes apparatus and techniques for grey scale operation for an electrophoretic display panel. The apparatus includes circuitry which operates with a timing generator which produces a plurality of different time duration output wave forms which are applied to the X and Y drive as associated with the display. In this manner, by applying a set of voltages for a given duration time interval, a display is provided which results in the incomplete removal of pigment from associated selected pixels. Hence, those pixels appear darker than surrounding pixels but not as dark as the pure dye solution as associated with the display. Thus, the amount of pigment removed and hence the darkness of each pixel is a function of the time duration during which the appropriate voltage is applied to the rows and columns of the display. The timing generator can cause different pixels as displayed to have different darknesses or grey scale values by varying the time during which the voltage is applied to the display. As will be further explained, grey scale operation at different shades of grey can also be provided on an electrophoretic display by means of area modulation. Area modulation can be used to shade either the foreground, the background or both the foreground and the background. Such electrophoretic displays, as other displays, portray information by writing in two different colors or shades of the same color. These of course can be referred to as black or white, although many other color combinations are available as indicated above. Thus in an electrophoretic display, the normal background color is the color of the pigment used in the display and the written characters and graphics are generated by removing pigment from the appropriate areas. In the reverse or inverse video mode the pigment is removed from the background while pigment is retained in the areas of the characters or graphics. This is the same difference, for example, between a negative and positive in photography. As will be explained, by performing area modulation by writing a pattern of either black or white pixels in either the background, foreground or both, permits generation of shades of grey. It is also understood that area modulation can be used with any relatively high resolution display to in fact provide a grey scale capability for the display.
It is therefore an object of the present invention to provide an electrophoretic display having grey scale capability.
It is a further object of the present invention to provide an electrophoretic display apparatus which has grey scale capability and which operates to modulate the area about each character or the area within each character on a display.